Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069; 5,593,838; and 5,603,351, all assigned to the David Sarnoff Research Center, Inc. and hereby incorporated by reference. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps include electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and them pumped into a bottom array of reaction cells. The array could be decoupled from the assembly and removed for incubation or analysis. When used as a printing device, the chemical reagent solutions are replaced by dispersions of cyan, magenta, and yellow pigment, and the array of reaction cells could be considered a viewable display of picture elements, or pixels, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of the paper fibers pulls the dye from the cells and holds it in the paper, thus producing a paper print, or reproduction, of the original scene.
One problem with this kind of printer is the rendering of an accurate tone scale. The problem comes about because the capillary force of the paper fibers remove all the pigment solution from the cell, draining it empty. If, for example, a yellow pixel is being printed, the density of the image will be fully yellow. However, in some scenes, a light, or pale yellow is the original scene color. One way to solve this problem would be to stock and pump a number of yellow pigments ranging from very light to dark yellow. Another way to solve the tone scale problem is to print a very small dot of dark yellow and leave white paper surrounding the dot. The human eye will integrate the white and the small dot of dark yellow leading to an impression of light yellow, provided the dot is small enough. This is the principle upon which the art of color halftone lithographic printing rests. It is sometimes referred to as area modulation of tone scale. However, in order to provide a full tone scale of colors, a high resolution printer is required, with many more dots per inch than would be required if the colors could be printed at different densities.
Another solution to the tone scale problem has been provided in the area of ink jet printers, as described in U.S. Pat. No. 5,606,351 by Gilbert A. Hawkins, the disclosure of which is hereby incorporated by reference. In an ink jet printer, the drop size is determined primarily by the surface tension of the ink and the size of the orifice from which the drop is ejected. The ink jet printer thus has a similar problem with rendition of tone scale. U.S. Pat. No. 5,606,351 overcomes the problem by premixing the colored ink with a colorless ink in the correct proportions to produce a drop of ink of the correct intensity to render tone scale. However, ink jet printers require a relatively high level of power to function, and they tend to be slow since only a few pixels are printed at a time (serial printing), in comparison to the microfluidic printer in which all the pixels are printed simultaneously (parallel printing).